In an uplink of LTE-Advanced, which is improved 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), a study is underway to introduce UL CoMP (Coordinated multiple point transmission and reception). CoMP is a technique aiming to improve mainly throughput of a terminal located in a cell edge, by performing transmission and reception with a terminal between a plurality of cells (base stations) in a coordinated manner.
In the case of UL CoMP, by receiving and combining a transmission signal from one terminal at a plurality of cells (base stations), reception quality is improved. At this time, within a group (hereinafter, referred to as “CoMP set”) of cells performing transmission and reception in a coordinate manner, terminal scheduling is also performed in a coordinated manner among a plurality of cells forming a CoMP set, in order to reduce influence of inter-cell interference.
On the other hand, LTE uses an SRS (Sounding Reference Signal) of uplink. Here, “Sounding” is referred to estimating channel quality, and an SRS is transmitted by time-multiplexing a specific symbol with data in order to mainly estimate CQI (Channel Quality Indicator) of uplink data channel.
LTE uses a ZC (Zadoff-Chu) sequence as an SRS. The characteristic of a ZC sequence includes that CS-ZC (Cyclic Shifted-ZC) sequences generated by cyclically shifting a ZC sequence of any ZC sequence number with a longer time length than the maximum propagation delay time are ideally orthogonal (inter-code interference is zero). However, ZC sequences having different ZC sequence numbers are not orthogonal, and cross-correlation (inter-code interference) occurs at a certain level of “1/ZC sequence length”. According to the above characteristic, LTE provides a ZC sequence group defining ZC sequence numbers for each transmission bandwidth available in cells, and one ZC sequence group is assigned to each cell (e.g. see Non-Patent Document 1). 30 of these ZC sequence groups are defined, and to reduce inter-cell interference, different ZC sequence groups are assigned to adjacent cells.
In order to improve reception quality in the above UL CoMP, accurate estimation of channel quality using an SRS is necessary. Therefore, at first, it is necessary to select a ZC sequence number for an SRS transmitted by a terminal to which UL CoMP is applied, that is, the terminal (hereinafter, referred to as “CoMP terminal”) where transmission signals are received and combined at a plurality of cells. As this selection method, two methods (selection method 1 and selection method 2) can be considered.
Selection method 1 is a method selecting, for an SRS of a CoMP terminal, a ZC sequence assigned to a cell (hereinafter, referred to as “serving cell”) that transmits control information such as scheduling information to the terminal. That is, in a serving cell of a CoMP terminal, a terminal (hereinafter, referred to as “Non-CoMP terminal”) to which UL CoMP is not applied uses the same ZC sequence for an SRS as a CoMP terminal.
Selection method 2 is a method selecting, for an SRS of a CoMP terminal, a ZC sequence of a ZC sequence number different from that of a ZC sequence to be used by a Non-CoMP terminal inside a CoMP set. That is, a ZC sequence belonging to a ZC sequence group (a ZC sequence group not used inside a CoMP set, that is, a ZC sequence group used outside a CoMP set) different from ZC sequence groups assigned to cells inside a CoMP set, is used in an SRS of a CoMP terminal.